ANALYSIS FOR THE ELEMENTS 123
to crack and traces of unreacted sodium will be destroyed by spon-
taneous burning without the dangers of a hydrogen explosion.
(Demonstration by instructor.) The cooled tube is now tapped
against the inner side of the beaker and the lower cracked part
allowed to drop into the water. The solid particles are broken
up with with a stirring rod, the solution heated to boiling and fil-
tered. The filtrate, which should be colorless, is reserved for the
subsequent tests.
A. Sulfur Test—To 1 cc. of the filtrate made slightly acid
with acetic acid, add a few drops of lead acetate reagent. A
black precipitate of PbS shows the presence of sulfur.
B. Nitrogen Test—Boil 3 cc. of the alkaline stock solution
for two minutes with 5 drops of FeSCXi, and 1 drop of FeCls solu-
tion. Cool and acidify carefully with HG1. The precipitate of
iron hydroxide should dissolve readily, otherwise the solution
should be warmed very gently. A clear yellow solution indi-
cates a negative nitrogen test; a blue precipitate indicates a posi-
tive test. A blue or greenish-blue solution suggests the presence
of nitrogen but indicates that the original sodium decomposition
may have been poor. The precipitate of Prussian blue shows up
best when it is collected and washed upon a white filter paper.
If iodine is present, the filter is washed with alcohol to dissolve
out the iodine. In the presence of sulfides, it will be advisable
to add enough FeSO4 solution to completely precipitate the sulfur
ions, filter off the FeS, and proceed as above.
Write equations illustrating the formation of Prussian blue.
C. Tests for Halogen, (a) General Test.—Acidify 2 cc. of
the stock solution with dilute HNOs and boil well to expel any
H:>S or HCN if present. Add AgNOs solution. A precipitate
denotes the presence of halogens. Also apply the Beilstein copper-
oxide-wire test to the original unknown.
(6) Tests for Bromine and Iodine in the Presence of Each
Other and the Other Halogens.—Acidify 2 cc. of the stock solution
with HoSO^ and boil gently to drive off EfeS. Add not more than
J cc. of carbon tetrachloride and finally a drop of a solution of
freshly prepared chlorine water. Shake after the addition of each
drop. If iodine is present, the carbon tetrachloride will be colored
purple. Continued additions of chlorine water will cause the
iodine color to disappear, due to the formation of the iodate, and
if bromine is present the carbon tetrachloride will become colored